I have one of these that has problems with loud intermittent pops and crackles on TV1 - 3, but no other channels.  They also occur with the sound muted, which leads me to suspect a specific software problem rather than any kind of hardware problem, since it only occurs on TV1, TV2, and TV3 (and not the time-shifted TV3+1 or any other channel), and happens even with the sound muted.  The only thing I can see that might affect this is that these three channels are present multiple times (presumably on multiple TPs) and all other channels are only present once, but that could be just coincidence.  Any ideas?